The events which occur during the assembly and maturation of murine leukemia virus (MuLV) particles are poorly understood. Two factors have prevented the analysis of these processes: (1) the lack of adequate methods for the identification of the structural components comprising the intermediates in virus synthesis; and, (2) the lack of methods for the synchronization of virus synthesis so that the steps in virus assembly and maturation can be placed in the proper temporal sequence. The development, in our laboratory, of a new method for immunoelectron microscopy and the availability of a series of temperature sensitive (ts) mutants of MuLV blocked in particular stages of assembly now enable us to systematically follow the course of assembly events which culminate in the release of mature virus particles. Immunoferritin labelling will be used to locate viral antigens in cells containing the ts mutants after thin sectioning for electron microscopy. The ferritin label as well as cellular and viral ultrastructure are observed simultaneously, allowing us to determine the intracellular (and intraviral) location of the virus structural components in each stage of particle assembly. We will also employ high resolution autoradiography to determine when the viral RNA is added to the forming virus particle. These studies will define the sequence of temporal events that occur during the assembly and maturation of MuLV particles.